The present invention relates to an image forming apparatus such as a printer or a copier and an image forming unit, and more particularly relates to a color image forming apparatus and an image forming unit in which a plurality of multistage type driven sections including image carriers are arranged, and in which when the driving power is transmitted from the driving source to the driven sections through a train of gears, the phase adjustment is correctly controlled corresponding to the eccentricity of the gears.
As an example of various types of image forming apparatuses such as a printer or a copier, an electro photographic type image forming apparatus has widely been known. In such an image forming apparatus, a latent image is formed on a photosensitive member as an image carrier by photo write-in, and the latent image is changed to a toner image (development), and the developed toner image is transferred and fixed onto a sheet.
In this type of image forming apparatuses, recently, there has been the request of image formation in full color, and further, it has widely come into practice.
Such color image forming apparatuses can be divided into two major categories: the single drum type and the multistage drum type (tandem type). In the single drum type, for 1 page of sheet, in order to transfer one over the other a total of four types of toners: the respective color toners of M (magenta: red dye) toner, C (cyanogen: greenish blue) toner, and Y (yellow: yellow color) as the three primary colors of the subtractive color mixture; and K (black: black color) toner used for printing letters or the like only, the printing (image formation) processing is separately performed for each toner. Therefore, the printing process is repeated four times for 1 page of sheet, and accordingly, the printing processing requires a long time.
On the other hand, since in the tandem type, four types of toners are transferred on a sheet in order, one over the other in 1 process, the tandem type has a speed of approximately four times the processing speed of the single drum type. Therefore, recently, color image forming apparatuses with the structure of the tandem type have been used in plenty.
In such an image forming apparatus, there are a lot of sections to be driven to rotate, and to these sections to be driven to rotate, the driving force is transmitted from a motor through a driving force transmission system composed of a train of gears. Here, as a part especially requiring accuracy in rotation, there is an image carrier drum.
However, if a driving force transmission system composed of a train of gears is used for driving to rotate each driven sections represented by these drums as mentioned above, the rotational irregularity because of the eccentricity of each gear occurs in the drums. The rotational irregularity like this is not a special problem in a monochrome printer using only one color of black toner, but in a color image forming apparatus (color printer) performing printing by applying one over the other 3 colors or four colors of color toners, a difference occurs in the position of the dots (toner image elements) applied one over the other on a sheet, if there is a rotational irregularity in the drums. Usually, the printing is performed by a density of dots of approximately nine pieces in 1 mm, and for example, even if there is a positional difference of 1/2 dot in the colors applied one over the other, a stripe pattern called moire occurs on the image surface, so that the quality of the image formed on the surface of a sheet may be extremely lowered.
However, since in any members, there is a limit in the manufacturing accuracy thereof, a plurality of driving gears corresponding to a plurality of three or four drums in a color printer or the like, are invariably accompanied by mechanically produced errors in shape or dimension. Accordingly, it cannot be avoided that a rotational irregularity occurs on the basis of errors in the drums driven thereby.
There are image forming apparatuses well known in Jpn. Pat. Appln. KOKAI Publication No. 61-156162 and Jpn. Pat. Appln. KOKAI Publication No. 9-179372, wherein considering the above-mentioned facts, that is, accepting the inevitable rotational irregularity, the image formation is performed while synchronizing the mutual relative image transfer positions in the image transfer sections of these plurality of drums at all times, and therefore, the gears of the same order (the same position) of the driving force transmission systems are molded by the same die to be used, in order to cancel the positional differences of the images applied one over the other.
In the image forming apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-156162, first, marks are given to the specified positions of the gears molded by the same mould or die, and from the marks, the gear ratio of a train of gears, and the clearance between the respective drums, the position where each drum is synchronized in the rotational phase at each image transfer position, is calculated, and on the basis of that, each drum gear is arranged. Consequently, the same rotational irregularity occurs in each drum in synchronism with the mutually corresponding image transfer position, but it is arranged that the transfer for each color is performed at the same position on the sheet, since the fluctuation of the moving speed of the peripheral surface relative to the image transfer position of each drum is each synchronized.
However, in the image formation, there are not only a demand for the multicolor printing, but also a demand for performing of one color printing of black, which is rather larger than the former. Accordingly, in the image forming apparatuses, there are a lot of apparatuses in which the mode can be shifted between a full color mode to perform the multicolor printing and a monochrome mode to perform one color (black) printing. In this case, usually, it is general that avoiding a method accompanied with a technical difficulty, and for easiness of achievement, even in the case of the monochrome mode, only the photo write-in driving to the photosensitive drum corresponding to the color system, is stopped, and the mechanical rotational driving is left to operate. Accordingly, the above-mentioned relative positions of the driving gears set by once performing the position fitting, do not get out of order.
However, if the mechanical driving of the color system is performed and the photosensitive drum of the color system is rotated even in printing of black only like this, since an initializing charged roller, a development roller, a cleaning blade and the like slidingly touch the photosensitive drum at all times, the members of the color system are exhausted by this sliding touch. Consequently, such a problem that though actually the color printing is not so much performed, the life of members is exhausted, occurs. The problem is solved tentatively if it is arranged that when the mode is shifted from the full color mode to the monochrome mode, both write-in driving and rotational driving are together stopped in the color system and only the black system is driven. However, if this is arranged, not only a large technical difficulty occurs in the change of the mechanical driving system, but also the following problem is derived: even if the mutually corresponding image transfer positions of the respective photosensitive drums are adjusted with efforts as mentioned above, the setting of the relative position does not coincide between the drum driving gear for black which is driven and the drum driving gear for the color which is stopped during that time, after the printing in the monochrome mode has finished.
Therefore, in the latter of Jpn. Pat. Appln. KOKAI Publication No. 9-179372, in order to solve the above-mentioned problem, in addition to the arrangement in the former of Jpn. Pat. Appln. KOKAI Publication No. 61-156162, a sensor is arranged to detect a mark of the gear. Then, two systems of driving force transmission systems which should be inevitably separated into the color and the monochrome, are separately driven, and when each sensor detects the mark, the driving force transmission system is stopped to perform positioning, so that the mutual position fitting of the two systems of driving force transmission systems may be performed, and consequently, it is arranged that when a color printing is performed next time, the synchronization of the two systems of driving force transmission systems can be achieved if these are together driven.
In the above-mentioned method of position fitting, the positioning of either of the two systems of driving force transmission systems is performed (by detecting a mark with a sensor) to stop the driving force transmission system, and next, the positioning of the other driving force transmission system is performed (similarly by detecting a mark with a sensor) to stop the driving force transmission system. By the way, generally, in the above-mentioned image forming apparatus, the sections requiring the largest torque as the mechanical torque are the drum in charge of the transfer section and the development section connected to that to be driven. For example, in a case where these are driven by a single motor together with other driven sections, 90% of the loads applied to the motor are produced in the development section and the transfer section. However, between the two systems of driving force transmission systems, the driving system joining the color printing is composed of 3 pieces of drums and the torque thereof is extremely large, but the driving force transmission system of the monochrome printing is composed of one piece of drum and the torque thereof is comparatively small. Then, in order to drive all of these at the same time, that is, in order to make it possible to correspond to the largest torque supposed to be necessary in the full color printing, a motor with a rated value to obtain a sufficient torque is used as the driving source.
The inertia (inertia, force of habit) of a motor producing a large torque like this is large corresponding to the magnitude of the produced torque. Therefore, as mentioned above, when the driving force transmission system with one piece of drum for the monochrome printing which is a driving force transmission system on one side, is driven and is stopped by a stop signal, it cannot immediately respond to the stop signal to stop instantaneously, and it stops after a somewhat long time has been elapsed, because of the above-mentioned large inertia.
Accordingly, as for the actual stop positions when the two systems of driving force transmission systems are separately driven and stopped, one position is a little ahead of the planned reference position, and the other is a little behind, so that a difference occurs between the respective positions. Therefore, such a problem that even if it is intended to perform synchronization for position fitting, actually, the two systems of driving force transmission systems mutually cause the positional difference, has been left.
By the way, the irregularity of rotational characteristics requiring the phase adjusting occurs not only in the train of gears of the driving system, but also in the photosensitive drum and the drum gear. In this case, if the photosensitive drum and the drum gear are made with an extremely high accuracy, the difference of the rotational phase does not occur, and therefore, it is only necessary to carry out the synchronization of the drum driving gears of the two systems of driving force transmission systems, as mentioned above. However, recently, it is common that the main section of the image forming members including the photosensitive drum, is unitized and is arranged in the housing of the image forming device in such a manner that attachment and removal are free. Accordingly, in order to make the photosensitive drum and the drum gear with a high accuracy, such a problem that the manufacturing cost of the unit rises considerably, is derived. For the problem, in the Jpn. Pat. Appln. KOKAI Publication No. 61-156162, a phase control means including such unitizing of the image forming members is not provided.